You will need to create an outbound rule/ prefix to dial out to PBX B, otherwise the call will not be routed. Please refer to the following link for more details on how to configure a bridge. https://www.3cx.com/docs/manual/connecting-pbx-bridges/
I have built the outbound rules for this location. This customer has multiple locations and the bridges are setup the same. This is the only one I get this error on.
I would run the firewall checker and check the IP blacklist. I would also check the configuration of the bridged connect. More precisely, the virtual extension number, Password and the Master/ Slave settings.